Fancy you see faces in the moon? Perhaps you'll notice their expressions tomorrow night, when the full moon is the biggest its been in nearly two decades. Referred to as a "supermoon," the lunar phenomenon occurs when our favorite space rock is in perigee, its closest orbital position to Earth. At this spot, the moon is 14 percent bigger, and 30 percent brighter than when it's farthest from Earth, at apogee, according to NASA’s video above. Further, each perigee isn’t always the same because of changes in the moon’s orbit. Tomorrow’s supermoon will be “just 221,566 miles (356,577 kilometers) away from Earth,” writes Andrew Fazekas in National Geographic. “The last time the full moon approached so close to Earth was in 1993, according to NASA,” he adds.

Still not convinced of this event's rarity? “The Moon is exactly 100 percent full only one instant a month, and that moment is very unlikely to line up with the exact moment of perigee,” writes Ethan Siegel, a visiting assistant professor of physics at Oregon’s Lewis and Clark College and author of the science blog, Starts With a Bang. “In other words, sometimes we see the Moon full near perigee, sometimes near apogee, and most of the time somewhere in-between.” Tomorrow's supermoon will be full less than an hour away from perigee. That’s "a near perfect coincidence that appears once every 18 years or so," said Geoff Chester of the U.S. Naval Observatory in Washington DC. "I'd say it's worth a look."

And if Japan’s recent earthquake and tsunami have you worried about a supermoon’s effect on Earth’s tides, don’t fret. “The 'Supermoon' will cause the highest high tides and the lowest low tides we've seen in a long time,” writes Siegel, “but only by about an inch.” And I’m guessing there won’t be any werewolves, either.